Commercial and residential buildings are often built on foundations comprising vertical perimeter walls of poured concrete. Since the vertical foundation walls are structural members which support the building, they are usually several feet in depth and function as beams bridging between footers or piers resting on bedrock or stable soil. It is common practice in such buildings to provide a basement, or ground floor, wherein at least a portion of the basement walls include the vertical foundation walls and wherein the basement floor is a poured concrete slab resting on the soil enclosed by the foundation walls. Typically, the foundation is constructed by first excavating a pit for the basement and digging trenches around the periphery of the pit for the foundation footers. Then forms are erected around the periphery of the pit and concrete for the foundation walls is poured into the forms. Depending on the load-supporting requirements of the foundation and the supporting soil, it is usually necessary to provide footers or piers of some type under the foundation walls.
A major problem with conventional construction in certain soil and climate conditions is that the location of the basement floor can be unstable due to movement of the underlying soil. This problem can be especially severe when the floor is simply a slab of concrete poured onto the surface of the soil which forms the floor of the excavation pit. For example, certain dense clay soils, such as bentonite, tend to dry out after excavation and then later absorb water and swell. This swelling causes the slab to move relative to the foundation walls and can generate large forces which are sufficient to crack or break the slab. In general, because the foundation walls must support the building, they are supported by solid ground or bedrock and therefore are very stable. However, when the basement floor is a relatively thin slab of concrete having a large surface area and resting on a large area of soil, it is highly vulnerable to movement due to expansion and contraction of the soil as water is absorbed and released by the soil. The relative motion between the slab and the walls can damage interior walls and therefore precludes the supporting of interior walls on the slab.
A second problem associated with a conventional poured concrete slab is the loss of heat through the slab. Since concrete is a poor thermal insulator, heat can be readily transferred through the slab and into the soil below when the slab is resting on the soil. This heat transfer can result in higher fuel consumption and heating costs for the building as well as a loss of thermal comfort due to radiant heat loss to the cold slab.
A variety of inventions have been made in the art of concrete slabs. For example, U.S. Pat. Nos. 2,881,501 and 3,358,960 disclose fiberboard forms for creating voids to reduce the weight of concrete slabs. U.S. Pat. No. 4,685,267 discloses a fiberboard box for creating a void in and under a concrete formation. U.S. Pat. No. 4,702,048 discloses a form, for cast in situ concrete slabs, comprising a sheet of material formed into a planar base with an array of upwardly convex hemispherical protrusions for creating voids in the concrete. U.S. Pat. No. 4,799,348 discloses a process for constructing pin point foundations and for using both recoverable and non-recoverable forms to produce a rigid ribbed slab resting on the pin point foundations. U.S. Pat. No. 5,224,321 discloses a method of installing prefabricated floor panels on temporary supports while pouring permanent concrete supports. U.S. Pat. No. 5,352,064 discloses a foundation having a moisture resistant spacer resting on expansive soil and supporting a flat surface on which a concrete slab can be poured. The spacer is designed to permanently deform if the soil expands after the concrete has hardened thereby preventing damage to the slab. This requires that the spacer be carefully designed to be strong enough to carry the weight of the slab during construction and weak enough to subsequently permanently deform or collapse under the force of expanding soil without damage to the slab.
Other inventions have attempted to solve only the heat transfer problem. U.S. Pat. Nos. 3,673,750 and 3,956,859 disclose concrete slabs poured on insulating material resting on the soil. U.S. Pat. No. 5,174,083 discloses a system for casting a floating slab with perimeter insulation.
Although each of the previously listed inventions has attempted to solve either the problem of slab movement due to soil expansion or the problem of heat loss through a slab, they have not provided a single solution to both of these problems. The present invention provides a lightweight rigid slab assembly which can be used over expansive soils without movement or damage, to itself or to interior walls resting on the slab, and which also provides thermal insulation to substantially reduce heat loss through the slab.